“filling in”の意味は?

名探偵シャーロック・ホームズを現代のイギリスに蘇らせたBBCのテレビドラマ『Sherlock』を観ながら、ETC英会話のトニー先生のレッスンを受けています。

今日はこんなダイアログから。

JOHN:　Have you talked to the police?
SHERLOCK:　Four people are dead, there isn’t time to talk to the police.
JOHN:　So why are you talking to me?
SHERLOCK:　Mrs Hudson took my skull.
JOHN:　So I’m basically filling in for your skull?
SHERLOCK:　Relax, you’re doing fine.

さて、“filling in”とはどのような意味なのでしょうか。

Sherlock Holmes has a skull, which he likes to talk to.
That comes from Shakespeare, you know that ”Alas, poor Yorick!”.
It’s very famous Hamlet, I think it is, where is talking to his brother skull.
It is a kind of play on that.
So he has a skull, which he likes to talk to.
So John, “why you are talking to me”.
And his housekeeper took away his skull.
So he couldn’t find his skull to talk to, which is why he was talking to John.
So that’s is of course.
So John says “filling in” in this situation, we use,,,
“Filling in”, OK, here we go again, the direct translation of “filling in” would be to repair something, You put a little, there is a hole in a wall.
So you get a little bit cement you fill in the hole.
But now we use it a lot for replacing somebody, in the place of.
So If I didn’t come to work today.
You would be filling in for me, my position.
I am filling in.
So John didn’t come to work today.
I am filling in for him.
So the skull wasn’t here.
So John was taking the place of the skull.
He was filling in for the skull.
So you always put filling in for whatever is missing.

skull 
頭蓋骨，どくろ

Alas, poor Yorick!
ああ, かわいそうなヨリック　(『ハムレット』シェークスピア作 より)
(*)参考リンク　
Alas, poor Yorick! – Hamlet by William Shakespeare
「哀れ、ヨリック」 – シェイクスピア『ハムレット』

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